The present invention relates to a method for manufacturing a primary preform for optical fibres by means of an internal plasma chemical vapor deposition (PCVD) process, the method including the steps of: i) providing a hollow glass substrate tube having a supply side and a discharge side; ii) supplying a gas flow in the interior of the hollow substrate tube via the supply side thereof, the gas flow including a main gas flow including at least one glass-forming gas and at least one secondary gas flow including at least one precursor for a dopant; iii) creating a plasma reaction zone in the interior of the hollow substrate tube by means of microwave radiation for effecting the deposition of glass layers on the inner surface of the hollow substrate tube, the reaction zone being moved back and forth along the longitudinal axis of the hollow substrate tube between a reversal point located near the supply side and a reversal point located near the discharge side of the hollow substrate tube, to obtain a substrate tube having glass layers deposited on its inner surface, and; iv) optionally subjecting the substrate tube having glass layers deposited on its inner surface obtained in step iii) to a collapsing treatment so as to form a solid primary preform.
The present invention relates generally to the field of optical fibres, and more particularly, to the field of manufacturing optical fibres by means of chemical vapour deposition. There are several types of chemical vapour deposition (CVD) known, such as outside vapour deposition (OVD), vapour axial deposition (VAD), modified chemical vapour deposition (MDVD) and plasma-enhanced chemical vapour deposition (PECVD or PCVD). Plasma-enhanced chemical vapour deposition (PECVD or PCVD) is a process used to deposit thin films from a gas state (vapour) to a solid state on a substrate. Chemical reactions are involved in the process, which occur after creation of a plasma of the reacting gases.
Generally, in the field of optical fibres, multiple thin films of glass are deposited on the inside surface of a substrate tube. The substrate tube is hollow to allow internal deposition. The substrate tube may be glass, preferably glass quartz (SiO2). Glass-forming gases (i.e., reactive gases including gasses for the forming of glass and optionally precursors to dopants) are introduced into the interior of the substrate tube from one end (called the “supply side” of the substrate tube). Doped or undoped glass layers (depending on the use of reactive gases with or without one or more precursors to dopants, respectively) are deposited onto the interior surface of the substrate tube. The remaining gases are discharged or removed from the other end of the substrate tube called the “discharge side” of the substrate tube. The removal is optionally carried out by means of a vacuum pump. The vacuum pump has the effect of generating a reduced pressure in the interior of the substrate tube, the reduced pressure generally having a pressure value ranging between 5 and 50 mbar.
Generally, the plasma is induced by the use of microwaves. Generally, microwaves from a microwave generator are directed towards an applicator via a waveguide, wherein the applicator surrounds the substrate tube. The applicator couples the high-frequency energy into a plasma that is generated inside the substrate tube. The applicator is moved reciprocally in the longitudinal direction of the substrate tube. Thus, the plasma formed, also called the “plasma reaction zone,” is also moved reciprocally. As a result of this movement, a thin glass layer is deposited onto the interior of the substrate tube with every stroke or pass.
Thus, the applicator is moved in translation over the length of the substrate tube within the boundaries of a furnace that surrounds the substrate tube and the applicator reciprocating within the furnace. With this translational movement of the applicator the plasma also moves in the same direction. As the applicator reaches the inner wall of the furnace near one end of the substrate tube, the movement of the applicator is reversed so that it moves to the other end of the substrate tube towards the other inner wall of the furnace. The applicator, and thus the plasma, travels in back and forth movement along the length of the substrate tube. Each back and forth movement is call a ‘“pass” or “stroke”. With each pass a thin layer of glass material is deposited on the inside of the substrate tube.
This plasma causes the reaction of the glass-forming gases (e.g. O2, SiCl4 and e.g. a precursor for a dopant, such as GeCl4 or other gases) that are supplied to the inside of the substrate tube. The reaction of the glass-forming gases allows reaction of Si (Silicon), O (Oxygen) and e.g. the dopant Ge (Germanium) so as to thus effect direct deposition of, for example, Ge-doped SiOx on the inner surface of the substrate tube.
Normally, a plasma is generated only in a part of the substrate tube, i.e., the part that is surrounded by the microwave applicator. The dimensions of the microwave applicator are smaller than the dimensions of the furnace and of the substrate tube. Only at the position of the plasma are the reactive gasses converted into solid glass and deposited on the inside surface of the substrate tube. Since the plasma reaction zone moves along the length of the substrate tube, glass is deposited more or less evenly along the length of the substrate tube.
When the number of passes increases, the cumulative thickness of these thin films, i.e. of the deposited material, increases, thus leading to a decrease in the remaining internal diameter of the substrate tube. In other words, the hollow space inside the substrate decreases with each pass.
The applicator and the substrate tube are generally surrounded by a furnace so as to maintain the substrate tube at a temperature between 900-1300° C. during the deposition process.
After the glass layers have been deposited onto the interior of the glass substrate tube, the glass substrate tube is subsequently contracted by heating into a solid rod (“collapsing”). The remaining solid rod is called a “primary preform.” In a particular embodiment, the solid rod or primary preform may furthermore be externally provided with an additional amount of glass, for example by means of an external vapour deposition process or direct glass overcladding (so-called “overcladding”) or by using one or more preformed glass tubes (so-called “sleeving”), thus obtaining a composite preform called the “final preform.” From the final preform thus produced, one end of which is heated, optical fibres are obtained by drawing on a drawing tower. The refractive index profile of the consolidated (final) preform corresponds to the refractive index profile of the optical fibre drawn from such a preform.
One way of manufacturing an optical preform by means of a PCVD process is known from U.S. Pat. No. 4,314,833 in the name of the present applicant. According to the process that is known from that document, one or more doped or undoped glass layers are deposited onto the interior of a substrate tube, using a low-pressure plasma in the glass substrate tube.
According to International application WO 99/35304 in the name of the present applicant, microwaves from a microwave generator are directed towards an applicator via a waveguide, which applicator surrounds a glass substrate tube. The applicator couples the high-frequency energy into the plasma.
The inventors have observed that these prior art PCVD processes lead to the deposition of soot (manifested as an opaque ring on the inner surface of the hollow substrate tube called “soot ring”) at the supply side of the primary preform. At the discharge side of the primary preform an area is observed having a higher doped silica, which is prone to cracking. Without wishing to be bound to any particular theory, it is assumed that such soot deposition takes place as a result of the relatively low intensity of the plasma in the region where the soot deposition takes place. It is further assumed that the temperature on the inner surface of the hollow substrate tube at the reversal points plays an important part in the formation of such a soot ring. The presence of such a soot ring has an adverse effect on the effective length of the preform. After all, the region of the soot ring in the substrate tube cannot be used for forming an optical fibre therefrom which meets the product specifications. Another drawback of the soot deposition is that there is a significant risk of fracture of glass layers, which means a loss of the total preform rod, which is undesirable.
Layer cracking is attributed to the fact that a high stress level will cause fracture at the location of any irregularities in the glass, which irregularities occur in particular in soot rings.
Since the current commercial trend in manufacturing of optical fibre preforms tends to go towards larger (thicker preforms), more passes or strokes will be required. This leads to tubes after deposition (before collapsing) having an even smaller internal diameter and even thicker soot ring. The problem of cracking, particularly in the vicinity of the supply side of the substrate tube is becoming more and more pressing.
This problem has been recognized previously and in the prior art several solutions have been proposed. Some of these solutions are discussed below.
One solution involves increasing the temperature of the PCVD furnace for subsequent depositions at either the supply side or the discharge side or both (depending on where the cracking occurs), when cracking is observed in the region of the soot ring during the collapsing treatment. This increase in temperature only partly addresses the problem. There are limits to the amount of temperature increase and the temperature difference between the middle part of the PCVD furnace and one or both ends (supply and/or discharge side).
Another solution is proposed in EP 1 988 064 from the present inventors. This document relates to the axial variation of the reversal point(s) during different phases (passes) of the glass deposition. In other words, the deposition of the soot ring is spread out over a larger area and hence the chance of cracking is reduced. For each phase of the deposition process (e.g. for the deposition of the core and for the deposition of the cladding), the reversal points at the supply side are moved, thereby providing a different placement of the soot ring. This method is effective, but could lead to a number of adjacent soot rings which may decrease the effective length of the preform.
Yet another solution is proposed in EP 1 801 081 by the present inventors. The solution involves the use of a so-called “insertion tube,” being a tube that is inserted at the supply side of the substrate tube inside of the hollow substrate tube. The effect is that the soot is partly deposited inside of the insertion tube and can easily be removed by the removal of the insertion tube. This method is very useful, but in some cases might lead to so-called “deposition oscillation,” which is undesirable.
Yet another solution is proposed in EP 2 008 978 by the present inventors. This method applies an etching step between two separate phases, during which etching step any non-uniformities are removed from the substrate tube. Even though this document discloses the interruption of the deposition process, the method according to EP 2 008 879 is different from the present invention. The etching is carried out by an etching gas. This method is very useful in a large number of applications. However, for the mere removal of a soot ring this method is labor intensive and costly.
Therefore, there is a need for an alternative solution to the problem discussed above.